Field of the Invention
The present invention relates to a method of manufacturing a single crystal by the Czochralski method (hereinafter referred to as CZ method) and, more particularly, to a method of measuring a melt surface position and a diameter of a single crystal in a pulling-up process of the single crystal.
Description of Related Art
The CZ method is one method for growing a silicon single crystal which becomes semiconductor materials. With the CZ method, a cylindrical single crystal is grown by slowly pulling up a seed crystal hanging on a wire, while coming in contact with melt in a crucible and solidifying the melt.
The melt inside the crucible is gradually reduced as the pulling-up process progresses. As such, in order to keep the thermal dose to the melt by the heater constant, it is necessary to raise the crucible so that the melt surface position is at a predetermined position relative to the heater and the heat-shield structure. This is because if the melt surface position relative to the heater and the like is not predetermined, the temperature history of the single crystal changes, crystal defects emerge and a single crystal of good quality cannot be produced.
For this reason, a method is proposed, where the melt surface position is optically measured while the single crystal is being pulled up and an amount of rise for the crucible is calculated (Japanese Patent Nos. 408950 and 4246561 and Japanese Patent Application Laid Open Nos. 2007-290906 and 2009-57216). Because this method ascertains the melt surface position directly, the margin of error is low, allowing the quality of the single crystal to improve.
For example, Japanese Patent No. 408950 proposes a method for calculating a center position of a single crystal and from the position of the fusion ring that emerges near the solid-liquid interface between the single crystal and the melt and measuring the surface position from this center position. In this method, first and second measuring lines are defined in an image of a fusion ring, which is detected by employing image detection. The first and second measuring lines are provided in front of the dipping position of the seed crystal in the necking process in a perpendicular direction and are separated by the first and second distances each from the dipping position. Then, the center position of the single crystal along the perpendicular direction in the image is calculated from the two intervals between the intersections of each measuring line and the fusion ring, on both sides, and from the first and second distances, and the melt surface position is measured based on this center position. This method is advantageous as a measuring method of melt surface position as it is unaffected by any inclination of the liquid surface. Particularly, in situations where the fusion ring can only be partially observed during the pulling-up process of the single crystal, the center position of the single crystal can be calculated with little computation and as a result, the melt surface position can be measured with high precision in comparison to conventional methods.
Additionally, in order to increase the manufacturing yield of silicon wafers, it is also crucial to limit variation in the diameter of single crystals. As a method of maintaining a constant diameter of the single crystals, there is a known method that involves detecting the diameter of the single crystal while it is being pulled up and based on the detected diameter, controlling the pulling-up speed and the heater's power supply (heater temperature). Additionally, in Japanese Patent Application Laid Open No. 2003-12395, for example, a method is described, where the diameter of a single crystal is maintained by detecting the diameter and center of the boundary between the silicon melt and the single crystal, from an image of the boundary taken by an image-capturing device, thereby determining the diameter and center position of the single crystal and adjusting the growth conditions based on these results.
According to the inventor's experiments, in the conventional method described above, it is necessary to calculate the center position of the single crystal from the position of the fusion ring in the captured image, but a problem was found, where if the melt adheres to the heat shielding body, which is placed above the melt, and the luminance distribution of the fusion ring changes, it is not possible to accurately calculate the center position of the single crystal, and it is also not possible to accurately determine the surface position and the diameter of the single crystal.
The adherence of melt to the heat shielding body tends to occur when depositing additional raw material. Even if solid raw material is charged in the crucible to its full capacity be forehand and then melted, an empty volume emerges due to a reduction in volume of the raw material. It is then possible to increase the raw-material melt amount in the crucible by further depositing additional solid raw material. By so doing, it is possible to effectively utilize the capacity of the crucible and increase productivity in single-crystals growth.
However, if agglomerated raw material is additionally deposited to the melt, the impact causes some of the melt to spray on and adhere to the surface of the heat shielding body. In instances of melt adherence originating from additional deposits of raw material, because the deposit direction of the raw material is predetermined, much of the melt tends to adhere to a particular area on the heat shielding body, close to the additional-deposit position. Furthermore, the agglomerated raw material first packed into the crucible can lose its balance and collapse during the initial melting and splatter, causing the melt to adhere to the heat shielding body. There is no regularity to this type of melt adherence and the melt does not adhere to any particular location on the heat shielding body. These causes comprise only one example and there is a variety of causes for the adherence of melt on the heat shielding body. When the melt adheres to the surface of the heat-shielding body, it affects a change in the luminance distribution of the fusion ring, which increases a measurement error of the surface position and the diameter of the single crystal.
It is possible to resolve measurement deviation due to melt adherence by replacing the heat shielding body. However, since melt adherence usually occurs during the raw-material melting process, it is not possible to make the replacement while the melting process is taking place. In addition and the manufacturing cost will increase if the frequency of replacement is high. It is therefore desirable to resolve measurement deviation due to melt adherence without replacing the heat shielding body.